> This page is for SDK, version Legacy.
> For other versions, use one of these documentation indexes:
> - Current (default): https://buildwithfern.com/learn/sdks/current/llms.txt
> - Legacy: https://buildwithfern.com/learn/sdks/legacy/llms.txt

> For clean Markdown content of this page, append .md to this URL. For the complete documentation index, see https://buildwithfern.com/learn/llms.txt.

# Adding custom code

> Learn how to add custom logic and methods to your Go SDK with Fern. Extend generated clients with helper functions and custom code.

> **You're viewing the legacy SDK docs**
>
> These pages cover SDK generation configured with `generators.yml`. New projects use `sdk-config.yml`, so see [the Current version of this page](/learn/sdks/generators/go/custom-code) for up-to-date guidance. To move an existing project, follow the [SDK Config migration guide](/learn/sdks/legacy/generators/migration-guides/sdk-config), which walks through `fern sdk migrate`, and review the [`sdk-config.yml` reference](/learn/sdks/reference/sdk-config).

> **Enterprise feature**
>
> This feature is available only for the [Enterprise plan](https://buildwithfern.com/pricing). To get started, reach out to [support@buildwithfern.com](mailto:support@buildwithfern.com).

This page covers how to add custom logic and methods to your Go SDK.

## Adding custom logic

To get started adding custom code:

> **Note**
>
> These steps cover adding a new custom file to the SDK. To preserve line-level edits to a generated file, use [Replay](/learn/sdks/legacy/overview/custom-code#replay) instead.

### Create a new file and add your custom logic

**`helper.go`**

```go title="helper.go"
func MyHelper() {
  fmt.Println("Hello World!")
}
```

### Add your file to `.fernignore`

**`.fernignore`**

```yaml {3} title=".fernignore"
# Specify files that shouldn't be modified by Fern

helper.go
```

### Consume the helper

Now your users can consume the helper function by importing it from the SDK.

```go
import "github.com/package/example"

example.MyHelper();
```

## Adding custom SDK methods

Fern also allows you to add custom methods to the SDK itself (e.g.
`client.my_method()` ) by inheriting the Fern generated client and then
extending it.

### Update `generators.yml` configuration

Name your Fern-generated client something like `BaseClient` to reflect that this client will be extended.

**`generators.yml`**

```yml {4} title="generators.yml"
- name: fern-go-sdk
    version: "..."
    config:
      clientName: BaseClient
```

### Import and extend the generated client

First, import the Fern generated base client and extend it. Then, add whatever methods you want.

**`client/my_client.go`**

```go title="client/my_client.go"
type MyClient struct {
  *Client // Embed the Fern generated client.
}

func NewMyClient(opts ...option.RequestOption) *MyClient {
  return &MyClient{
    Client: NewClient(opts...),
  }
}

func (m *MyClient) MyHelper() {
  fmt.Println("Hello World!")
}
```

### Update `.fernignore`

Add the `client/my_client.go.` to `.fernignore`.

**`.fernignore`**

```diff title=".fernignore"
+ client/my_client.go
```

### Consume the method

Instead of constructing the generated client, your users will want to construct the extended client.

**`main.go`**

```go title="main.go"
import exampleclient "github.com/package/example/client"

client := exampleclient.NewMyClient():
```

Now your users can consume the helper function by importing it from the SDK.

```go
client.MyHelper()
```